Variable width band-pass filter



K. WILHELM 2,205,075

VARIABLE WIDTH BAND-PASS FILTER Filed July 10, 1937 INVENTOR KARLW/LHELM BY 7KZW ATTORNEY Patented June 18, 1940 UNITED STATES VARIABLEWIDTH BAND-PASS FILTER Karl Wilhelm, Berlin, Germany, assignor toTelefunken Gesellschaft fiir Drahtlose Telegraphic m. b. H.,-Berlin,Germany, a

Germany corporation of Application July 10, 1937, Serial No. 152,926 InGermany July 10, 1936 3 Claims.

The present invention relates to band pass filters of adjustable bandwidth.

The object of the present invention is to provide means for varying theband width ofthe filter without incidentally and undesirably varying themid frequency of the transmitted band.

It is known in the art to vary the band width of a filter made up ofcoupled resonant circuits by varying the coupling between said circuits.It is further known to vary a coupling between two resonant circuits byvarying an impedance in a link circuit utilized for coupling between twosuch circuits. This method is desirable in that there is no necessityfor mounting one or more coils movable for the purpose of couplingvariation, but this method as in the past suffered from the drawbackthat the tuning of the filter, or more precisely, the position of themid frequency of the transmitted band is thereby altered. It is believedthat the reason for this effect upon tuning is that the effectiveinductance in each of the resonant circuits coupled by a link circuitdepends upon the impedance of the link circuit and hence in variation inthe link circuit impedance effects the resonant frequency of thecircuits coupled together'by the link. In accordance with the inventiontwo pairs of tuned circuits are utilized, each pair being variablycoupled by means of a variable link circuit, but the link circuits areso composed that the detuning effect of varying one link circuit is inthe opposite direction from the detuning effect resulting from variationof the other link circuit. As a consequence the overall band-passcharacteristic is merely spread out in both directions equally as theband width is increased. When the band width is considerably increasedthree maxima and two minima of transmission appear and in accordancewith another object of the invention the transmission is made moreuniform over a wide frequency range by employing still another filterarrangement having two maxima of transmission coinciding with theaforementioned minima so that signals traversing all three filtercomponents successively will be substantially uniformly transmitted overa relatively high band.

In the accompanying drawing Fig. 1 illustrates the detuning effect in asingle filter stage, Fig. 2 shows a two stage filter in which the twostages have oppositely directed detunling effects and in which the bandwidth controls are interconnected for uni-control. Fig. 3 representstransmission characteristics of the composite filter of Fig. 2, and Fig.4 represents the transmission characteristics of a well known type ofcoupled circuit filter adapted to be employed in connection with afilter of the type shown in Fig. 2 for the purpose of improvinguniformity of signal transmission over a wide band.

Referring specifically to Fig. 2 the lefthand portion thereof shows afilter comprising a pair of resonant circuits coupled by mutualinductance M to an extent too small to produce bandpass characteristicsand additionally coupled in the same sense by way of a link circuitincluding a variable series resistance B. When R is very large thetransmission through the filter is sharply selective as shown in curve Iof Fig. 1. The mid frequency of this curve is indicated by the letter a.When however R is very small the effective coupling between the resonantcircuits is sufiiciently close to produce the familiar double humpedresponse curve as shown in curve 4 of Fig. l. The reaction of the linkcircuit upon the associated resonant circuit is such as to reduce theeffective inductance in each of the resonant circuits thereby causingthe mid frequency of the transmitted band to increase from point a ofFig. 1 to point 0. This effect could be 'compensated by a variation inthe electrical constants of the resonant circuits through a uni-controlof such constants with the variation of R. In the preferred form of theinvention however a second filter is utilized in cascade with the firstfilter, as shown in the right hand side of Fig. 2. In this second filterthere is a small capacity coupling c between two resonant circuits andan auxiliary capacity coupling in the form of a link circuit'including avariable resistance W. The coupling 0 is a weak coupling adapted toproduce a sharp selectivity characteristic similar to curve I of Fig. 1when the resistance W is so large that the link coupling is ineffective.On the other hand when W is made very small so that the circuits aresufficiently tightly coupled to produce a double humped characteristicthe effect of the link circuit upon the associated resonant circuits isto reduce their natural frequencies so that the transmissioncharacteristic is the mirror image of curve 4 of Fig. 1 with respect topoint (1.. Hence if resistance R and W are uni-controlled the overalltransmission characteristic of Fig. 2 Varies from curve I of Fig. 3 tocurve 4 of Fig. 3 as the resistances are simultaneously reduced from alarge value to a very small value. It is seen that there is no shift ofthe mid frequency of the transmitted band in this case.

It will be observed that when a wide band is to be transmitted curve 4of Fig. 3 departs from the ideal band pass characteristic in that thereare two dips in transmission within the band. In order to furtherimprove the transmission characteristic, in accordance with theinvention, a third filter element of any suitable design having thetransmission characteristic shown in Fig. 4 may be employed in cascadewith the two elements shown in Fig. 2. The two transmission maxima ofthis third filter are chosen to coincide with the two transmissionminima or dips of curve 4 of Fig. 3 so that the overall transmissioncharacteristic of the entire system is rendered substantially uniform.

While I have indicated an arrangement for providing a variable bandwidth with incidental detuning, it will be understood that any othertype of filter having incidental detuning may be equally well employed.I therefore do not wish to be limited except in accordance with thefollowing claims.

What I claim is:

1. A variable band Width filter comprising two portions each includingpair of resonant coupled circuits, resistive means for varying thecoupling between the pair of resonant circuits of one portion andincidentally shifting in one direction the mid frequency of thetransmitted band of said filter portion, means for varying the couplingbetween the pair of resonant circuits of the second portion andincidentally shifting in the opposite direction the mid frequency of thetransmitted band of said second filter portion, and means forsimultaneously varying both of said means for varying said couplings insuch a relation as to maintain constant the mid frequency of thetransmitted band of said variable band width filter.

2. In combination, a variable band width filter comprising two portions,means for varying the coupling between elements of one portion andincidentally shifting in one direction the mid frequency of thetransmitted band of said filter portions, means for varying the couplingbetween elements of the second portion and incidentally shifting in theopposite direction the mid frequency of the transmitted band of saidsecond filter portion, means for simultaneously varying both of saidvarying coupling means in such a relation as to maintain constant themid frequency of the transmitted band, and a second filter including twomaxima of response, both of said filters being arranged for successivetransfer of signals therethrough, the said maxima of response of Saidsecond filter being arranged to coincide with the minima of response ofsaid first named filter whereby to produce substantially uniformtransmission of signals over the maximum band width of said first namedfilter.

3. In a variable width band-pass filter, a first pair of tuned circuitsresonant to the same frequency, a link circuit including a resistancefor coupling said tuned circuits, the variation of said resistance fromhigh to low values resulting in respectively a highly selective responseand a wide band response with incidental shift in one direction of themid frequency of the transmitted frequency band, a second pair of tunedcircuits resonant to the same frequency as the first mentioned circuits,means including a resistance for coupling said second pair of tunedcircuits, the variation of said resistance from high to low valuesresulting in respectively a highly selective response and a wide bandresponse with incidental shift of the mid frequency of the transmittedband in a direction opposite to that of the frequency shift of the firstpair of tuned circuits, and means for simultaneously varying saidresistances.

KARL WILHELM.

